Using a master document and sub documents, I want to change the page setup to be more analogous to a paperback book than a 8.5/11 paper.
How can I change this for the master document without having to change it in the sub documents?
If the style (any kind) have the same name on both documents, the one on the master will take precedence over the one on the subdocument so just edit the master document style and everything will (should) be Ok.
I know that. I’m asking how to do that. On the Master Document → Format Page is greyed out, so I can’t actually edit the style at all to begin with.
Use the style editor instead. Press F11 to open the styles and formatting editor, select page styles (the fourth button on top) and edit the page style you need to change.
My advice: NEVER use the Format → Page shortcut to change page format, always go to the style editor and modify the corresponding style.